Perfluorinated compounds (PFCs) are widely applied in industrial manufacturing, such as chemical industry, textile, paint, leather, synthetic detergent, cookware manufacturing (such as non-stick pans), paper packaging material for food, etc., because of their unique C-F chemical bond (having high bond energy, high redox potential, etc.). PFCs have been produced and used for more than 50 years. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) are typical PFCs, and are widely used as surfactant, anti-fouling agent, additive, foam fire extinguishing agent, polymeric emulsifier, pesticide, etc. PFOA and PFOS are chemically stable and hardly biodegradable, and have frequently been detected in water, atmosphere, and biosphere throughout the world. PFOA can be absorbed by human body in various ways such as ingestion, inhalation, and skin contact, and may induce various diseases such as cancer, hepatomegaly, etc. Thus, PFOA poses a serious threat to human health.
At the beginning of the 21st century, as PFCs were frequently detected in human blood samples all over the world, global pollution of PFCs and impact of PFCs on human health have attracted attention of governments and scientific community. In December 2002, at the 34th Joint Meeting of the Chemical Committee held by the Organization for Economic Co-operation and Development (OECD), PFOS was defined as a substance that is persistent in the environment, bioaccumulative, and harmful to human beings. According to “the United States Toxic Substances Control Act” (2003), PFOS was included in the list of banned chemicals. In May 2009, PFOS and salts of PFOS, together with the precursor of PFOS, i.e., perfluorooctane sulfonyl fluoride, were officially added to Annex B (Restriction) of “Stockholm Convention on Persistent Organic Pollutants”. Based on the above measures, production and use of PFCs have been constrained.
Currently, technologies for treating PFCs in a solution mainly include pyrolysis, electrochemical oxidation, sonochemistry, photocatalytic degradation, and the like. PFCs are from extensive sources in the environment, but exist in the environment with very low concentrations, usually ranging from nanogram level to microgram level in a water body, and up to milligram level in a heavily-polluted water body. When the above degradation technologies are directly used for treating water, energy consumption is high and efficiency is low, so the above degradation technologies are difficult to be applied at a large scale. A feasible technical route is to pretreat polluted water, to concentrate PFCs in the water body and thus reduce the amount of water for treatment.
Existing technologies for concentrating PFCs in a solution mainly include reverse osmosis and ion-exchange resin method. However, generally, reverse osmosis has a single-stage water production rate of less than 80%, and has a concentration ratio of less than 5. To improve the concentration ratio, multi-stage reverse osmosis has to be used, but cost of water treatment can be multiplied. Although the ion-exchange resin method using weak-base anion-exchange resins can have a high concentration ratio for PFCs in a solution, a large amount of chemical reagents, such as diluted aqueous ammonia or an organic acid, are needed for elution. This process is very time-consuming with complicated subsequent processing, and is expensive.
In this invention, floc generated by electroflocculation, such as ferric hydroxide or aluminum hydroxide, has a large specific surface area. PFCs in a solution can be quickly adsorbed onto the surface of the generated floc. By research, applicants have found that the floc generated by electroflocculation can adsorb and remove PFCs by hydrophobic interaction and hydrogen bonding. According to such characteristics, the PFCs adsorbed on the surface of the floc can be re-released into a solution by a centrifugal force, in order to achieve the purpose of concentrating and separating PFCs and, at the same time, achieve dewatering and harmless treatment of the sludge. Such a process can concentrate the PFCs in the solution by several hundred times. Therefore, using such a technology, PFCs at a low concentration in a solution can be concentrated, to reduce the cost of subsequent treatment of the PFCs solution and improve the efficiency of the treatment. Because electroflocculation has a low cost of treatment and produces a small amount of sludge, applying electroflocculation at a large scale is economically feasible.
To date, there has been no technical report related to removing PFCs in a solution by concentration and separation using electroflocculation and centrifugation techniques.